Most of the commercially available known HID lamps are used for several purposes, for example low power metal halide lamps are used for indoor lighting applications. HID lamps have electrodes without preheating features. The lamps have to start reliably with cold electrodes, and perform the specified electrical and light characteristics both initially and through their life after reaching steady-state operation temperatures. These requirements set different and often contradicting conditions to the electrode design.
In addition to these requirements, dimming is a great advantage in applications in which light output of the lamp has to be adjusted to some reduced value, or actual conditions allow reduction of lamp power and energy consumption.
The most important types of HID lamps are the high-pressure mercury, high-pressure sodium, metal halide, discharge automotive, and special type (projection, studio, etc.) lamps. Continuous mode dimming is particularly important in the low wattage range of HID lamps intended for interior lighting or possibly for vehicle lighting.
Conventional electrode designs allow dimming down only about to 60% of the nominal wattage, which makes them unusable for certain applications.
According to the solutions disclosed in U.S. Pat. Nos. 2,887,603 and 2,951,171, a special pair of electrodes is used in a lamp. One of them comprises a thorium-oxide emission material in a conical cavity. The other one discloses an electrode with coil that provides a nest for the emission material in order to reduce the loss rate thereof and consequently to lengthen the life of lamp.
Use of thorium-oxide as emission material in the form of tablet or pellet is indicated in U.S. Pat. No. 3,619,699 that relates to electrodes of discharge lamps. Penetration of the arc terminus into an electrode cavity is assisted by vapor breathing of the emission material, which injects plasma ingredients into the cavity during AC re-ignition after current zero. Such breathing is very desirable in high-pressure low-current lamps. Breathing is favored by a cavity, which has a depth not substantially greater than the terminus penetration depth. High temperatures deep within the cavity are advantageous, and are achieved by providing enhanced thermal coupling between the forward end of the cavity member and the cooler radiation shield surrounding it, and also by thermal insulation between the sides of the cavity member and the cooler shield. Disposing the emission material within the lower portion of the cavity favors deeper terminus penetration. Projection of the radiation shield beyond the cavity member is avoided because such projection would favor formation of a spot mode arc terminus on the shield.
This teaching provides hints to use a spiral member on the tip of the electrode, but the spiral member does not really form a cavity. The spiral member is used to make a reservoir for the emission material. The reservoir is not completely filled with the emission material, but an element that holds this material in place fills up the cavity completely. It is also complicated to manufacture such electrodes due to the emission material insert and large number of electrode components.
Another solution is known from the published patent application US 2006/0238127, in which the discharge vessel has a first and a second mutually opposed neck-shaped portion provided with a pair of electrodes, each of which is tubular over its entire length. The rod and spiral combination is declared in this document as having several drawbacks such as hardly controllable thermal contact between them. Therefore the electrode is manufactured with a tungsten tube mounted onto a tungsten rod with an intermediate member. A tungsten tube of small diameter is very expensive and requires non-conventional electrode manufacturing technology that may further increase the costs of production and may involve dimension-control issues primarily at electrodes of submillimeter size.
There is a need for HID lamps with cavity electrodes that require simple and cost effective manufacturing technology.
Recently emerging requirement is that HID lamps are dimmable with a continuous and wide range of wattage. There is a particular further need for meeting this requirement by suitable electrode structure.